Fast to slow megathrust slip and fault strength at seismogenic depths of the Cascadia subduction zone

In the past two decades, a new type of plate boundary fault deformation mode was observed in global subduction zones. These transient slip events, termed episodic slow slip events (SSEs), reflect the quasi-static stress release at the downdip end of the seismogenic zone. Investigating the mechanism of SSEs, especially on their relation with the in situ subduction zone structure, enriches our knowledge of the megathrust fault. By incorpoerating the 3D curved Cascadia subdcution slab geometry into the rate-and-state fault model, I simulate the spontaneous recurrences of SSEs and study their spatiotemporal evolutions. The modeled SSEs capture the major characteristics revealed by the GPS observations. The along-strike distribution of SSE is inversely related to the fault local dip and local strike angle of the SSE depths, suggesting a strong geometrical influence. Besides the GPS-detectable fast-spreading phase, each SSE cycle consists of a deep pre-SSE preparation (nucleation) and a post-SSE relaxation (healing) phase, which may be the driving mechanism for the inter-ETS (Episodic Tremor and Slip) tremor activity that is discovered in Cascadia. This finding also suggests detailed process of SSEs along the Cascadia margin can be imaged by a GPS network with higher-resolution detection limits. . The relationship between SSE along-strike variation and upper plate the heterogeneity is not yet well understood. Using a 800 km long fault model, I reproduce the segmentation of SSE with the parameters that are constrained by the non-volcanic tremor epicenters and gravity anomaly of the overlying plate. The results indicate that not only the fault fricitonal properties, but also the entire loading from topography and density of the overlying plate can affect the ocurrence of SSEs. The Cascadia Initiative (CI) expedition deployed dense ocean bottom seismometer (OBS) arrays and provided a great opportunity to monitor the megathrust fault status in central to southern Cascadia. The focal mechanism solutions and stress inversion based on small-to-intermediate earthquakes during 2012-2013 and 2014-2015 expeditions reveal a stress rotation from the continental crust to the oceanic mantle near the Mendocino Triple Junction. The fault shear strength scales with a subjective mantle strength assumed in the inversion. The megathrust fault shear strength can be no higher than 50 MPa and the resolved friction coefficients are in the range of 0-0.2, regardless of the assumed mantle strength. The weakened megathrust fault from the seismogenic zone to the SSE depths possibly results from the dehydration of the oceanic slab.

@article{id2378,
  author = {Li, Duo},
  journal = {Ph.D. Thesis},
  language = {en},
  title = {Fast to slow megathrust slip and fault strength at seismogenic depths of the Cascadia subduction zone},
  year = {2018},
}

%O Journal Article
%A Li, Duo
%J Ph.D. Thesis
%G en
%T Fast to slow megathrust slip and fault strength at seismogenic depths of the Cascadia subduction zone
%D 2018